In the Region of the Summer Stars

In the Region of the Summer Stars by Stephen R. Lawhead is the start of an epic fantasy adventure series set in a realm rich in imagination and intrigue, and steeped in the wonder of the Celtic Otherworld.
The isle of Eirlandia has been ravaged by the barbarian Scálda, who seek to conquer all. The High King has called for the warring clans and tribes of the Tuatha DeDanann to set aside their feuds and unite to fight to save their nation.
Conor is the first-born son of the Celtic king Ardan mac Orsi. He should have been battlechief, but a birthmark casts him as unclean and that honor has fallen to his younger brother Liam. Conor is resigned to his fate, but wishes he could do something to earn his tribe’s respect.
Sometimes wishes take unexpected turns—when Conor is wrongly accused of theft and cast out of his tribe, he embarks on a dangerous mission to not only prove his innocence but to expose the treachery that led him on this path. He also seeks the ethereal beauty he saw being abducted by the Scálda. Convinced that she is no mortal woman, but one of the faéry, Conor must find and rescue her.
Because he knows that if the Scálda gain the secret of faéry magic, they will conquer his homeland.